Union: Airlines outsourcing plane maintenance

The TWU study found that 40 percent of the maintenance for the major airlines was outsourced. Southwest sends many of its 737 airplanes to El Salvador for work, the group said.

Since the incident last Friday, where Southwest Flight 812 from Phoenix to Sacramento, Calif., landed in Yuma, Ariz., shortly after take off, the airline has maintained its planes are safe.

"Our highest priority is the safety of our Employees and Customers," Southwest Vice President Mike Van de Ven said this week in a statement. "Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft. What we saw with Flight 812 was a new and unknown issue."

None of the 118 passengers or five crew members aboard Flight 812 last Friday was injured.